1. Field of the Invention
This invention pertains to the general field of interferometric metrology. In particular, it pertains to an automated method for periodically re-focusing the reference mirror of the interferometer to account for environmental changes during repeated interferometric measurements.
2. Description of the Prior Art
The interferometric objectives used in the art are usually off-the shelf components (such as produced by Nikon and Olympus) that allow manual adjustments to refocus the objective's reference mirror as needed to account for drift caused by environmental factors, such as temperature, humidity and vibrations. Thus, the process of recalibration of an interferometric instrument is slow and unsuited for online testing of parts for quality-control purposes, where the measured parameters are expected to be essentially the same during repeated measurements.
As is well understood in the art, an interferometric measurement, whether conducted with white-light so-called vertical-scanning interferometry (VSI) or with narrow-band phase-shifting interferometry (PSI), produces acceptable results only if the optical path difference (OPD) between the reference and test arms is zero when the sample surface is in focus. This condition is especially critical for measuring small features, such as the roughness of machined metal parts and the air bearing surface (ABS) of computer head sliders. Therefore, when environmental factors affect the initial calibration of the interferometer that ensures this OPD/focus condition, the quality of the measurements deteriorates and testing is no longer reliable. If the location of zero OPD is shifted away from the objective's focal point (a condition that occurs when the reference mirror is out of focus), the modulation produced by the interferometric scan is reduced with the attendant loss of measurement precision, which in turn reduces the ability to control whether the parameter of interest is maintained within the desired specification.
This invention describes a mechanism and a process for the automated adjustment of the reference mirror in the objective so that re-focusing can be carried out automatically at predetermined periods during the operation of the instrument.